1. Field
The present invention relates generally to wireless communications systems, and more particularly to down-converting narrow band radio frequency (RF) signals.
2. Description of Related Art
In a frequency division multiple access (FDMA) analog cellular system, such as an Advanced Mobile Phone System (xe2x80x9cAMPSxe2x80x9d) analog cellular radio system, the available frequency spectrum is divided into radio channels, e.g., pairs of transmit and receive carrier frequencies, each of which corresponds to a message transmission channel. The bandwidth of each transmit and receive frequency channel is narrowband, generally 25-30 kHz. The cellular service area in the FDMA system is generally divided into a plurality of cells, each cell having a set of frequency channels selected so as to minimize co-channel interference between cells.
Further, expanding mobile telephone market and the advantages of digital processing have been driving many advancements in digital cellular applications. One commonly used communication standard among mobile phones is Code Division Multiple Access (CDMA).
Direct downconversion schemes have been proposed which accomplish the RF-to-baseband downconversion using a single stage. In this instance the LO frequency and RF carrier can be matched in frequency and the resultant signal is produced at baseband. This technique uses a homodyne receiver to the down convert in a single stage. However, this single downconversion technique, referred to as Zero Intermediate Frequency (ZIF) downconversion, produces unwanted inter-modulation products. For example, it produces a zero frequency DC component which adversely affects the data content of the baseband signal. This DC component may be removed using digital techniques if the analog ZIF baseband signal is first converted to the digital domain by Analog to Digital (A/D) conversion. The DC component can then be mathematically removed via bit manipulations, for example. Thus, the technique of ZIF downconversion is attractive due to its simplicity.
A need exists for making receivers incorporating such a direct-downconversion scheme for wide-band RF signals, such as those found in CDMA systems, compatible with narrow-band signals, such as those signals employed in connection with the AMPS analog cellular radio system, for example.
A method and device for converting at least one narrow band RF signal, being suitable for transmission between at least one communications device suitable for receiving wide-band RF signals and at least one base station, to baseband is disclosed. The invention includes directly down-converting a signal spectrum including the at least one RF narrow-band signal to baseband such that the at least one narrow-band RF signal results at a low intermediate frequency (IF); and, digitally phase rotating the down-converted signal spectrum such that the at least one narrow-band RF signal is phase rotated from the low-IF to baseband.